The application of peroxidase immunocytochemical techniques to human tissue utilizing antibodies to apolipoproteins (developed in our branch), was successfully demonstrated in our laboratory during the past year. Human jejunal tissue was analyzed for the presence of apolipoproteins A-I, A-II, and B in the epithelial cell biopsies from subjects, and patients with familial HDL deficiency (Tangier Disease) and abetalipoproteinemia. Apolipoproteins A-I and A-II were present in normal tissues and in intestinal samples from Tangier Disease. These studies are consistent with the view that both apoA-I and apoA-II are synthesized in patients with Tangier Disease. ApoA-I and apoA-II were also present in the intestinal cells of patients with abetalipoproteinemia. These studies have been of major importance in our analysis of the sites of apolipoprotein synthesis in normals and has provided insight and raised interesting questions about apolipoprotein metabolism in dyslipoproteinemic subjects. Recently, steps have been undertaken to improve the technique to expand the tissues being investigated to include coronary arteries, liver, and lymphoreticular tissue.